The present invention is directed to liquid-load measurements and in particular to sensors of the vibrating-diaphragm type.
Many approaches have been taken to making liquid-load measurements. One approach particularly well suited to some applications is that of determining the loading on a vibrating diaphragm that has been placed in contact with the liquid. The mass of liquid that loads the diaphragm affects the diaphragm's response to flexural excitation. The loading inferred from the diaphragm's vibration can be used to determine the depth of a liquid of known density or the density of a liquid of known depth.
U.S. Pat. No. 5,035,140 to Daniels et al., for instance, describes a sensor arrangement in which a member potentially loaded by liquid in a tank is caused to vibrate, and the resultant vibration is measured to determined whether it is consistent with the member's being loaded by the liquid. Such a sensor is employed to determine whether liquid in a tank has reached a level at which the sensor is mounted.
Although the Daniels et al. sensor presumably performs adequately for the indicated purpose, it is not well suited to providing a relatively accurate liquid-load measurement throughout a continuous load range. For that purpose, U.S. Pat. No. 5,345,811 to Alexandrovich et al. describes an approach that can be used to measure the density of fuel in an aircraft tank. The Alexandrovich et al. arrangement infers the density from the natural vibration frequency of a diaphragm as loaded by the fuel, with which it is in contact. To achieve the desired accuracy, Alexandrovich et al. employ processor-based calibrations to determine coefficients in an equation relating the density to the natural frequency. With these coefficients, a processor calculates the density of the tested fluid from the observed frequency.
To minimize the effects that ambient-pressure variations would have on the measurement, Alexandrovich et al. mount and excite the diaphragm in a special fashion. Specifically, the diaphragm extends through a sealed slit in the tank wall, and the diaphragm is so excited as to assume vibration in a mode that has a node at the wall location. This allows both faces of the diaphragm to be exposed to the fuel so as to cancel out pressure effects.